1. Field of the Invention
This invention relates to analog-to-digital converters, and more particularly, to an analog-to-digital converter having a minimal number of voltage comparing circuits.
2. Description of the Prior Art
Prior art analog-to-digital converters can generally be classified as either parallel-type converters or serial-to-parallel type converters.
Parallel-type analog-to-digital converters which produce n bit digital outputs generally require (2.sup.n -1) comparing circuits or comparators connected in parallel. An analog input voltage is supplied to the (2.sup.n -1) voltage comparators, and the outputs from the comparators are supplied to an encoder which then produces the n digit output.
In a serial-to-parallel type analog-to-digital converter having an output of m upper bits and n lower bits, the input voltage is supplied to a first stage of (2.sup.m -1) voltage comparing circuits or comparators, just as in the parallel-type analog-to-digital converter, to generate the m upper bits. The m upper bits are then supplied to a digital-to-analog converter and reconverted to an analog voltage. The reconverted analog voltage is then subtracted from the analog input voltage and the difference is supplied to a second stage of (2.sup.n -1) voltage comparators to derive the n lower bits.
In the parallel-type analog-to-digital converter, (2.sup.n -1) voltage comparators are required when the analog input voltage is converted to a digital output of n bits. A large number of circuit elements are required for such an analog-to-digital converter, and the resulting integrated circuit is large and has a correspondingly large power consumption.
In the serial-to-parallel analog-to-digital converter, only (2.sup.m +2.sup.n -2) voltage comparators are used when the output has m+n bits. The chip size and power consumption are reduced as compared to the parallel-type analog-to-digital converter. However, a serial-to-parallel analog-to-digital converter requires a digital-to-analog converter. If an error occurs between the first and second stages of the converter, the error is repeated in the digital output. For example, with a monotonically increasing voltage V.sub.in, if an error occurs between the first and second stages so that the output from the first stage is reduced, the digital output from the second stage will be reduced, and the digital output of the converter will not increase monotonically.